Algae volumes on rocks predicted to double

BY PENNY WARDLE

TrustPower's proposed Wairau hydroelectric power scheme could double the amount of algae growing on rocks in the affected reach of river, aquatic biologist Brian Coffey, of Whangamata, told the Environment Court in Blenheim yesterday.

The hearing is being held after an earlier decision by a Marlborough District Council-appointed panel to grant the company resource consent was appealed.

Periphyton algae supported fish and bottom-dwelling animals in the Wairau River, TrustPower expert witness Dr Coffey said. Too many floods limited periphyton volumes, while too few could result in blooms which could degrade the river habitat.

The presence of periphyton was good for the productivity of species that lived in the river, but in the right conditions, growth could reach a nuisance level, he said.

The scheme could cause a doubling of periphyton growth by slowing flow and increasing consecutive low-flow days, Dr Coffey said.

However, the risk would be reduced by the scheme being shut down during floods, which naturally cleared periphyton growth, and planned closure of the intake for 24 hours of flushing, when river flows increased to 150 cubic metres per second (cumecs) after an extended low summer flow period.

"The additional 40-cumec flush appears very likely to result in the removal of ageing periphyton mats from the diversion reach," he said.

"With these mitigation measures in place, I am satisfied that the potential effects of the scheme on periphyton growth will not result in a significant adverse effect on water quality or macroinvertebrate communities," he said.

Fish & Game lawyer Maree Baker suggested that while the frequency of nuisance periphyton growth might not change, reduced flows could cause the algae to reach nuisance levels earlier and for longer.

In the summer of 2005-2006, Dr Coffey observed a periphyton bloom develop after 268 days with no flushing flow. The weed extended from above the Branch River confluence to Marchburn, covering about 25 per cent of the wetted river, he said.

Flows of about 250 cumecs were required to shift young growth and up to 200 cumecs, once periphyton had aged and was more easily sloughed off rocks.

This disproved a general guideline that three times median flow was adequate to remove the weed, he agreed with commissioner Alex Sutherland.

Dr Coffey said the river probably reached its maximum carrying capacity for the weed during this bloom, as eventually the weed shaded itself out and roots died. With the TrustPower scheme in place, the infestation would have been no worse.

Ms Baker said that there were four negative effects of periphyton – on amenity values for people enjoying the river; on macroinvertebrate habitat; on oxygen levels; and the potential for toxic blooms.

High-density periphyton could kill fish as a result of a drop in oxygen levels, Dr Coffey agreed with Ms Baker.

However, this was unlikely on the Wairau River, which had deep channels which would remain free of the weed.

The risk of fish death would increase if water temperatures rose as a result of reduced flows because water had been removed from the river, he conceded.

Post-scheme, conditions in the Wairau would also be more favourable for didymo, said Dr Coffey. While the invasive waterweed was present in the river, it had not yet been found in the reach affected by the scheme.

When questioned, Dr Coffey said that with the use of effective and well-maintained screen cleaners to remove periphyton at power-station intakes and canal culverts, there should be no need to use aquatic herbicide for weed control.

If the aquatic herbicide diquat was used, he did not expect that it would contaminate groundwater.

- The Marlborough Express